JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year UNDERGROUND MINING TECHNOLOGY Previous Question Paper (February - 2022 - R16)

The formation of coal seams is a complex geological process known as coalification, which transforms organic plant matter into coal over millions of years. This process occurs in several distinct stages:

1. Accumulation of Plant Debris: Coal formation begins in vast, swampy environments where dense vegetation, such as ferns, trees, and mosses, grows. As these plants die, they fall into stagnant, waterlogged basins. The water prevents complete decay by limiting oxygen exposure, which inhibits the activity of aerobic bacteria and fungi.
2. Peat Formation: The submerged plant material undergoes partial decomposition by anaerobic bacteria, resulting in the formation of peat. Peat is the precursor to coal and consists of a dark, fibrous, and spongy mass of organic matter.
3. Burial and Sedimentation: Over geological time, the peat deposits are covered by layers of sediments such as sand, silt, and clay, often due to changes in sea levels or river flooding. As these layers accumulate, the weight of the overlying strata exerts significant pressure on the peat.
4. Coalification (Metamorphism): As the depth of burial increases, the material is subjected to rising temperatures (geothermal gradient) and increasing pressure. This process, known as coalification, drives off water and volatile gases (such as methane and carbon dioxide), progressively increasing the carbon content.
5. Stages of Coal Rank: The transformation proceeds through specific ranks based on the intensity of heat and pressure:
   • Lignite: The lowest rank, characterized by high moisture and low carbon content.
   • Sub-bituminous and Bituminous: Further compression and heating remove more moisture and volatiles, increasing the energy density and carbon concentration.
   • Anthracite: The highest rank, formed under intense tectonic pressure and heat, resulting in a hard, shiny, and high-carbon fuel.

The thickness and continuity of a coal seam depend on the duration of the swamp environment, the rate of plant growth, and the stability of the basin during the depositional phase. Tectonic activity may later fold or fault these seams, influencing their accessibility for mining.

The bord and pillar method is a primary underground mining technique used for bedded deposits like coal. It involves the systematic development of a network of intersecting roadways, known as bords or headings, which create a grid-like pattern of rectangular or square blocks of coal called pillars. These pillars serve as structural support to maintain the stability of the overlying strata (roof) during the extraction process.

The mining operation is typically divided into two distinct phases:

1. Development: The coal seam is opened by driving narrow galleries in two perpendicular directions. This phase extracts only a small percentage of the coal (typically 15-30%) to establish access and ventilation while leaving the majority of the coal in the pillars to support the roof.
2. Depillaring (Extraction): Once the development of a panel is complete, the pillars are systematically extracted or 'robbed' in a retreat sequence, moving from the boundaries of the panel back toward the main access roads. This allows the roof to cave in behind the working face in a controlled manner.

The primary technique used in bord and pillar mining is Cyclic Mining, often executed through the Drill-Blast-Load-Support cycle. This involves:

• Drilling: Creating blast holes in the coal face using hand-held or mobile drills.
• Blasting: Using explosives to fragment the coal face.
• Loading: Utilizing mechanical loaders or Load-Haul-Dump (LHD) machines to transport the broken coal to the transport system.
• Support: Installing roof bolts, steel props, or cross-bars immediately after extraction to ensure the safety of the working area.

In modern operations, this technique is increasingly replaced or supplemented by Continuous Mining, which utilizes a Continuous Miner machine. This machine uses a rotating cutting head to shear coal directly from the face without the need for drilling and blasting, significantly increasing production rates and safety by eliminating the use of explosives at the face.